A method for predicting microannulus of oil and gas well cement ring interface under multiple alternating loads

Abstract

A kind of oil and gas well cement ring interface micro annular space prediction method under multiple alternating loads, characterized in that, based on the test results of cement stone cyclic triaxial experiment under simulated working condition, the relationship between cement ring plastic strain and cycle number is established by combining plastic accumulation theory, based on interface coordination deformation theory, the cement ring interface stress (casing-cement ring interface and cement ring-formation interface) after unloading of alternating load is calculated using casing-cement ring-formation combination body elastic-plastic model, combined with the cement ring interface radial cementing strength of "casing-cement ring-formation" combination body physical test under simulated working condition in laboratory, the cement ring interface cementing state under multiple alternating loads is judged, and the quantitative calculation of cement ring interface micro annular space size is realized.The present application is suitable for the technical field of oil and gas drilling and production engineering.

Description

TECHNICAL FIELD

[0001] The patent relates to the technical field of oil and gas drilling engineering, and particularly relates to a method for predicting microannulus of cement sheath interface of oil and gas well under multiple alternating loads. BACKGROUND

[0002] The cement sheath of deep shale gas and tight gas unconventional horizontal wells is subjected to extreme service conditions, such as large displacement, high pump pressure (>100 MPa), severe alternating temperature (>150 DEG C), pressure and thermal-mechanical coupling effect generated by large-scale hydraulic fracturing of deep (well depth >4500 m) shale gas and tight oil and gas unconventional horizontal wells. Severe alternating temperature, pressure and their coupling effect can easily lead to cement sheath integrity failure, resulting in microannulus of cement sheath interface (including casing-cement sheath interface and cement sheath-formation interface), and causing continuous annular pressure and other safety problems.

[0003] The existing calculation and measurement methods for the microannulus of the cement sheath interface mainly include experimental method and theoretical method. The experimental method mainly measures the microannulus size by connecting a probe at the casing wall and monitoring the casing displacement during the cement sheath curing stage through the probe displacement, or qualitatively analyzes whether the microannulus is generated by using an ultrasonic imager; for example, the patents CN110080716A and CN110208503A measure the microannulus during the cement sheath curing stage by using the probe displacement, but cannot measure the microannulus generated in the later stage due to temperature / pressure change; the patent CN201710192740 qualitatively judges the interface cementation condition and whether the microannulus is generated under the temperature / pressure change by using the ultrasonic imager, but cannot calculate the accurate microannulus size. In terms of theory, the generation and development of the microannulus of the cement sheath interface under alternating temperature / pressure are analyzed by establishing an elastoplastic mechanics model of the cement sheath combination, but it can only be used for the first alternating action and cannot calculate the microannulus size of the interface under the multiple alternating temperature / pressure actions.

[0004] Therefore, in order to overcome the deficiencies of the prior art, in combination with theoretical analysis and experimental test, the present application provides a method for predicting the microannulus of the cement sheath interface of the oil and gas well under multiple alternating loads. The method can quickly and quantitatively calculate the microannulus size of the cement sheath interface (including casing-cement sheath interface and cement sheath-formation interface). The method can provide a theoretical basis for the mechanical properties of the cement sheath interface of the oil and gas well, the cement sheath integrity and the cementing optimization design. SUMMARY

[0005] The application aims to provide a method for predicting micro annulus of cement ring interface of oil and gas well under multiple alternating loads, so as to solve the technical problem of quantitative evaluation of micro annulus of cement ring interface (including casing-cement ring interface and cement ring-formation interface) under actual simulation working conditions.

[0006] To achieve the above-mentioned purpose, the method for predicting micro annulus of cement ring interface of oil and gas well under multiple alternating loads adopts the following technical scheme:

[0007] A method for predicting micro annulus of cement ring interface of oil and gas well under multiple alternating loads, which mainly comprises the following steps:

[0008] Step 1: preparing experimental samples for testing mechanical properties of casing-cement ring interface and cement ring-formation interface: (1) using field well structure and cement slurry system, simulating actual temperature and pressure curing to prepare casing-cement ring-formation combination;

[0009] Step 2: measuring mechanical properties of cement ring interface: (1) testing radial cementing strength σ bc of casing-cement ring interface; (2) testing radial cementing strength σ bf of cement ring-formation interface;

[0010] Step 3: preparing experimental samples for testing mechanical properties of cement stone: using field cement slurry system, simulating actual temperature and pressure curing to prepare standard cement stone samples (diameter 2.5 cm, height 5 cm);

[0011] Step 4: testing uniaxial and triaxial mechanical properties of cement stone: (1) testing stress-strain relationship curve of cement stone under uniaxial and triaxial states; (2) calculating cement ring elastic modulus E s , Poisson's ratio μ s , cohesive force C s and internal friction angle φ s for mechanical model calculation of casing-cement ring-formation combination;

[0012] Step 5: calculating cyclic triaxial peak deviatoric stress: (1) establishing elastoplastic mechanical model of casing-cement ring-formation combination under temperature, pressure and their coupling; (2) using field casing and formation parameters and cement ring elastic modulus E s , Poisson's ratio μ s , cohesive force C s and internal friction angle φ s measured by triaxial experiment of cement stone in laboratory, combining with the established elastoplastic mechanical model of casing-cement ring-formation combination to calculate maximum radial compressive stress Qs ;

[0013] Step 6: Test the cyclic triaxial mechanical properties of cement paste: Select the maximum radial compressive stress Q at the casing-cement ring interface s The cyclic triaxial compressive stress Q is determined as the peak deviatoric stress of the cement paste cyclic triaxial test. s stress-strain relationship of cement paste;

[0014] Step 7: Based on the results of cyclic triaxial test of cement paste, establish the plastic strain ε of cement paste dj Functional relationship f0(ε dj ,j), prepare for subsequent model prediction, where j = 1, 2, 3, ..., represents the number of alternating load loading and unloading;

[0015] Step 8: Calculate the radial stress at the casing-cement sheath interface after each alternating load unloading: Combine the elastic-plastic mechanical model of the casing-cement sheath-stratum combination established in step 5 and the functional relationship f0(ε dj ,j), calculate the radial stress P of the casing-cement sheath interface after each alternating load unloading csj (Pull is positive and pressure is negative);

[0016] Step 9: Determine whether the radial bonding failure occurs at the casing-cement sheath interface after the alternating load is unloaded: If the radial tensile stress P at the casing-cement sheath interface after the jth alternating load is unloaded csj For the first time, it is greater than the radial bonding strength σ of the casing-cement sheath interface bc , then the casing-cement sheath interface fails, otherwise, it does not fail;

[0017] Step 10: Calculation of the micro-annulus size at the casing-cement sheath interface: (1) After the casing-cement sheath interface fails, calculate the casing outer wall displacement u after the alternating load is unloaded. con0 and cement sheath inner wall displacement u sin0 , the size of the micro-annulus at the casing-cement sheath interface is: d cs0 =|u con0 -u sin0 |; (2) After the micro-annulus at the casing-cement sheath interface is generated, if the number of alternating loads continues to increase, the size of the micro-annulus at the casing-cement sheath interface will be: d csk =d cs0 +u smik , where k = 1, 2, 3, ..., represents the number of alternating load loading and unloading after the micro-annulus at the casing-cement sheath interface is generated; u smik It represents the plastic cumulative displacement in the radial direction of the cement sheath inner wall caused by the continuous increase in the number of alternating loads after the micro-annulus at the casing-cement sheath interface is generated;

[0018] Step 11: Calculate the radial stress at the cement sheath-stratum interface after each alternating load unloading: Combine the elastic-plastic mechanical model of the casing-cement sheath-stratum combination established in step 5 and the functional relationship f0(ε dj ,j), calculate the radial stress P of the cement sheath-stratum interface after each alternating load unloading sfj (Pull is positive and pressure is negative);

[0019] Step 12: Determine whether the radial bonding failure occurs at the cement sheath-stratum interface after the alternating load is unloaded: If the radial tensile stress P at the cement sheath-stratum interface after the jth alternating load is unloaded sfj For the first time, it is greater than the radial bonding strength σ of the cement sheath-stratum interface bf , then cement sheath-stratum interface fails, otherwise, no failure occurs;

[0020] Step 13: Calculation of the micro-annulus size at the cement sheath-stratum interface: (1) After the cement sheath-stratum interface fails, calculate the displacement u of the cement sheath outer wall when the alternating load is unloaded. son0 and the displacement of the inner wall of the formation u fin0 , the size of the micro-annulus at the cement sheath-stratum interface is: d sf0 =|u son0 -u fin0 |; (2) After the micro-annulus is generated at the cement sheath-stratum interface, the number of alternating loads continues to increase, and the size of the micro-annulus at the cement sheath-stratum interface is: d sfk =d sf0 +u smok , where k = 1, 2, 3, ..., represents the number of alternating load loading and unloading after the micro-annulus at the cement sheath-stratum interface is generated; u smok It represents the plastic cumulative displacement in the radial direction of the outer wall of the cement sheath due to the continuous increase in the number of alternating loads after the micro-annulus at the cement sheath-formation interface is generated.

[0021] The present invention has the following advantages:

[0022] Based on the elastic-plastic theory, the plastic accumulation theory, and the cement paste cyclic triaxial experiment, the relationship between plastic strain and the number of cycles is established, and the size of the micro-annulus at the cement sheath interface (including the casing-cement sheath interface and the cement sheath-formation interface) in the casing-cement sheath-formation combination under simulated working conditions is accurately predicted. This provides a new method for the optimization design of cement slurry systems and construction process parameters for oil and gas well cementing, as well as the evaluation of cement sheath integrity. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the technical roadmap of the present invention.

[0024] Figure 2 The uniaxial test results of cement paste in the examples are shown.

[0025] Figure 3 Cement stone triaxial test results in the example.

[0026] Figure 4 Cement stone cyclic triaxial test results in the example.

[0027] Figure 5 Relationship between cement stone plastic strain and cycle number in the example.

[0028] Figure 6 Casing-cement sheath interface radial stress and cement sheath-formation interface radial stress after unloading of different alternating load numbers in the example.

[0029] Figure 7 Casing-cement sheath interface microannular space development curve during unloading of the alternating load when the casing-cement sheath interface cementation fails in the example.

[0030] Figure 8 Cement sheath-formation interface microannular space development curve during unloading of the alternating load when the cement sheath-formation interface cementation fails in the example. DETAILED DESCRIPTION

[0031] In order to have a clearer understanding of the technical features, objectives and beneficial effects of the present application, the technical solutions of the present application are described in detail below with reference to the drawings, but it should not be understood as limiting the implementable scope of the present application.

[0032] Step one: preparing experimental samples for testing the mechanical properties of the casing-cement sheath interface and the cement sheath-formation interface: using the well structure and cement slurry system of a certain well in the southwest oilfield of China, casing-cement sheath-formation combination bodies were prepared under the curing condition of 120℃*90MPa, two groups (group A and group B), group A was used for testing the radial cementation strength of the casing-cement sheath interface, and group B was used for testing the radial cementation strength of the cement sheath-formation interface.

[0033] Step two: testing the mechanical properties of the casing-cement sheath interface and the cement sheath-formation interface, the test results show that the radial cementation strength of the casing-cement sheath interface is 0.53MPa, and the radial cementation strength of the cement sheath-formation interface is 0.29MPa.

[0034] Step three: preparing experimental samples for testing the mechanical properties of the cement stone: using the well structure and cement slurry system of a certain well in the southwest oilfield of China, standard cement stone samples (diameter 2.5cm, height 5.0cm) were prepared under the curing condition of 120℃*90MPa, three groups (group A, group B, group C), each group had three parallel samples.

[0035] Step four: testing the uniaxial and triaxial mechanical properties of the cement stone: (1) testing the uniaxial mechanical properties of group A cement stone, as shown in the attached Figure 2 (2) Test the triaxial mechanical properties of cement paste in group B, as shown in the attached Figure 3 (3) Combined with the uniaxial and triaxial test results, the elastic modulus E of cement paste is calculated. s is 4.47GPa, Poisson's ratio μ s is 0.1, cohesion C s is 5.77 MPa and the internal friction angle φ s The angle is 30°, which is used for calculating the mechanical model of the casing-cement sheath-formation combination.

[0036] Step 5: Calculate the peak deviatoric stress of cyclic triaxial stress: (1) Establish an elastic-plastic mechanical model of the casing-cement sheath-stratum combination under the effects of temperature, pressure and their coupling; (2) Use the field casing, formation parameters and the cement sheath elastic modulus E measured by the indoor cement stone triaxial test to calculate the peak deviatoric stress of the casing-cement sheath. s , Poisson's ratio μ s , cohesion C s and the internal friction angle φ s Combined with the established elastic-plastic mechanical model of casing-cement sheath-stratum combination, the maximum radial compressive stress Q of the casing-cement sheath interface under field conditions is calculated. s 15MPa;

[0037] Step 6: Test the cyclic triaxial mechanical properties of cement paste: Select the maximum radial compressive stress Q at the casing-cement ring interface s As the peak deviatoric stress of the cement paste cyclic triaxial test, the cyclic triaxial stress-strain relationship of the cement paste in group C was tested;

[0038] Step 7: Establish the relationship between the plastic strain of cement paste and the number of cycles: Based on the results of the cyclic triaxial test of cement paste, calculate the plastic strain ε of cement paste dj The relationship between f0(ε dj ,j), the results are as shown in the attached Figure 5 As shown;

[0039] Step 8: Calculate the radial stress at the casing-cement sheath interface after each alternating load unloading: Combine the elastic-plastic mechanical model of the casing-cement sheath-stratum combination established in step 5 and the cement sheath plastic strain ε measured in step 6 dj Relationship with the number of cycles f0(ε dj ,j), calculate the radial stress P of the casing-cement sheath interface after each alternating load unloading csj (Pull is positive and press is negative), the results are as shown in the attached Figure 6 As shown;

[0040] Step 9: Determine whether the radial bond failure of the casing-cement sheath interface occurs after the alternating load is unloaded: After the fourth alternating load is unloaded, the radial tensile stress (0.55 MPa) at the casing-cement sheath interface is greater than the bonding strength (0.53 MPa) at the casing-cement sheath interface for the first time. Therefore, after the fourth alternating load is unloaded, the casing-cement sheath bonding failure occurs.

[0041] Step 10: Calculation of the micro-annular gap size at the casing-cement sheath interface: (1) When the fourth alternating load is unloaded, the displacements of the casing outer wall and the cement sheath inner wall are as shown in the following figure. Figure 7 As shown in the figure, the micro-annular gap size d at the casing-cement sheath interface is calculated. cs0 =4.97μm; (2) After the micro-annulus at the casing-cement sheath interface is generated, the alternating load is continuously loaded and unloaded for 7 times, and the accumulated plastic displacement u of the inner wall of the cement sheath is smi7 is 13.52 μm. Therefore, after 11 cycles of alternating loading and unloading, the micro-annular gap size d at the casing-cement sheath interface is cs7 18.49μm;

[0042] Step 11: Calculate the radial stress at the cement sheath-stratum interface after each alternating load unloading: Combine the elastic-plastic mechanical model of the casing-cement sheath-stratum combination established in step 5 and the cement sheath plastic strain ε established in step 6 dj Relationship with the number of cycles f0(ε dj ,j), calculate the radial stress P of the cement sheath-stratum interface after each alternating load unloading sfj (Pull is positive and press is negative), the results are as shown in the attached Figure 6 As shown;

[0043] Step 12: Determine whether the radial bond failure occurs at the cement sheath-formation interface after the alternating load is unloaded: After the ninth alternating load is unloaded, the radial tensile stress (0.30 MPa) at the cement sheath-formation interface is greater than the cement sheath-formation interface bond strength (0.29 MPa) for the first time. Therefore, after the ninth alternating load is unloaded, the cement sheath-formation bond failure occurs.

[0044] Step 13: Calculation of the micro-annulus size at the cement sheath-stratum interface: (1) When the 9th alternating load is unloaded, the displacements of the cement sheath outer wall and the formation inner wall are as shown in the following figure. Figure 8 As shown in the figure, the micro-annulus size d at the cement sheath-stratum interface is calculated. sf0 =2.19μm; (2) After the micro-annulus at the cement sheath-stratum interface is generated, the alternating load is continuously loaded and unloaded twice, and the accumulated plastic displacement u of the outer wall of the cement sheath is smo2 is 1.15 μm; therefore, after 11 cycles of alternating loading and unloading, the size of the micro-annulus at the cement sheath-stratum interface is d sf2 It is 3.34μm.

[0045] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention should be covered by the scope of protection of the present invention.

Claims

1. A method for predicting micro-annulus at the cement sheath interface of oil and gas wells under multiple alternating loads, characterized in that: The method mainly comprises the following steps: Step 1: Prepare experimental samples for testing the mechanical properties of the casing-cement sheath interface and the cement sheath-formation interface: Using the on-site wellbore structure and cement slurry system, simulate the actual temperature and pressure curing to prepare the casing-cement sheath-formation combination; Step 2: Measure the mechanical properties of the cement sheath interface: (1) Test the radial bonding strength σ of the casing-cement sheath interface bc ; (2) Test the radial bonding strength σ of the cement sheath-stratum interface bf ; Step 3: Prepare experimental samples for cement paste mechanical properties testing: Use the on-site cement slurry system to simulate actual temperature and pressure curing to prepare standard cement paste samples with a diameter of 2.5 cm and a height of 5 cm; Step 4: Test the uniaxial and triaxial mechanical properties of cement paste: (1) Test the stress-strain curve of cement paste under uniaxial and triaxial conditions; (2) Calculate the elastic modulus E of cement paste s , Poisson's ratio μ s , cohesion C s and the internal friction angle Used for calculation of mechanical model of casing-cement sheath-formation combination; Step 5: Calculate the peak deviatoric stress in cyclic triaxial test: (1) Establish an elastic-plastic mechanical model of the casing-cement sheath-stratum combination under the effects of temperature, pressure and their coupling; (2) Use the field casing and formation parameters and the cement sheath elastic modulus E measured by the indoor cement stone triaxial test to calculate the peak deviatoric stress. s , Poisson's ratio μ s , cohesion C s and the internal friction angle Combined with the established elastic-plastic mechanical model of casing-cement sheath-stratum combination, the maximum compressive radial stress Q at the casing-cement sheath interface under field conditions is calculated. s ; Step 6: Test the cyclic triaxial mechanical properties of cement paste: Select the maximum radial compressive stress Q at the casing-cement ring interface s The cyclic triaxial compressive stress Q is determined as the peak deviatoric stress of the cement paste cyclic triaxial test. s stress-strain relationship of cement paste; Step 7: Based on the results of cyclic triaxial test of cement paste, establish the plastic strain ε of cement paste dj Functional relationship f0(ε dj ,j), prepare for subsequent model prediction, where j = 1, 2, 3, ..., represents the number of alternating load loading and unloading; Step 8: Calculate the radial stress at the casing-cement sheath interface after each alternating load unloading: Combine the elastic-plastic mechanical model of the casing-cement sheath-stratum combination established in step 5 and the functional relationship f0(ε dj ,j), calculate the radial stress P of the casing-cement sheath interface after each alternating load unloading csj , wherein the cement sheath interface radial stress P csj The sign of is defined as: positive value in tension state, negative value in compression state; Step 9: Determine whether the radial bonding failure occurs at the casing-cement sheath interface after the alternating load is unloaded: If the radial tensile stress P at the casing-cement sheath interface after the jth alternating load is unloaded csj For the first time, it is greater than the radial bonding strength σ of the casing-cement sheath interface bc , then the casing-cement sheath interface fails, otherwise, it does not fail; Step 10: Calculation of the micro-annulus size at the casing-cement sheath interface: (1) After the casing-cement sheath interface fails, calculate the casing outer wall displacement u after the alternating load is unloaded. con0 and cement sheath inner wall displacement u sin0 , the size of the micro-annulus at the casing-cement sheath interface is: d cs0 =|u con0 -u sin0 |; (2) After the micro-annulus at the casing-cement sheath interface is generated, if the number of alternating loads continues to increase, the size of the micro-annulus at the casing-cement sheath interface will be: d csk =d cs0 +u smik , where k = 1, 2, 3, ..., represents the number of alternating load loading and unloading after the micro-annulus at the casing-cement sheath interface is generated; u smik It represents the plastic cumulative displacement in the radial direction of the cement sheath inner wall caused by the continuous increase in the number of alternating loads after the micro-annulus at the casing-cement sheath interface is generated; Step 11: Calculate the radial stress at the cement sheath-stratum interface after each alternating load unloading: Combine the elastic-plastic mechanical model of the casing-cement sheath-stratum combination established in step 5 and the functional relationship f0(ε dj ,j), calculate the radial stress P of the cement sheath-stratum interface after each alternating load unloading sfj , wherein the cement sheath interface radial stress P sfj The sign of is defined as: positive value in tension state, negative value in compression state; Step 12: Determine whether the radial bonding failure occurs at the cement sheath-stratum interface after the alternating load is unloaded: If the radial tensile stress P at the cement sheath-stratum interface after the jth alternating load is unloaded sfj For the first time, it is greater than the radial bonding strength σ of the cement sheath-stratum interface bf , then cement sheath-stratum interface fails, otherwise, no failure occurs; Step 13: Calculation of the micro-annulus size at the cement sheath-stratum interface: (1) After the cement sheath-stratum interface fails, calculate the displacement u of the cement sheath outer wall when the alternating load is unloaded. son0 and the displacement of the inner wall of the formation u fin0 , the size of the micro-annulus at the cement sheath-stratum interface is: d sf0 =|u son0 -u fin0 |; (2) After the micro-annulus is generated at the cement sheath-stratum interface, the number of alternating loads continues to increase, and the size of the micro-annulus at the cement sheath-stratum interface is: d sfk =d sf0 +u smok , where k = 1, 2, 3, ..., represents the number of alternating load loading and unloading after the micro-annulus at the cement sheath-stratum interface is generated; u smok It represents the plastic cumulative displacement in the radial direction of the outer wall of the cement sheath due to the continuous increase in the number of alternating loads after the micro-annulus at the cement sheath-formation interface is generated.

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